Indicators of phthalein series are widely utilized as acid-alkali indicators which are colorless or almost colorless on an acidic side but show a clear tint on an alkaline side.
The present invention continues a variety of researches to develop a series of erasable inks which can be erased by the action of water, heat, an acid or the like. At this time, he has made researches to develop erasable into using indicators of phthalein series as main ingredients. In case of indicators of phtahlein series, it is apparent that the colors are erasable with an acid. It has now been found with interest that images written with these indicators, such as letters or patterns, are erased when the indicators are diluted with water or when the images are dried after the lapse of a definite period of time. However, it has also been found that problems arise in case of coloring the indicators of phthalein series, because the use of a strong alkali rather renders the color so faded as to make the indicators impossible to be used as inks, and on the other hand, the use of a weak alkali imparts only a weak tint to the indicators thereby making them impossible to be used as inks capable of imparting a clear tint.